It is often very desirable to determine the power necessary to operate a machine. This may be done by means of instruments called dynamometers. The prony brake is one of the most simple and familiar examples of the dynamometer.

Figure 32 represents one type of prony brake in which a fixed band of leather, or rope, is in contact with a portion of the circumference of a pulley or drum A. The band has one end attached as shown at B, while the weight C is hung at the other end.

The formula to find the foot-pounds per minute is:

3.1416 X diam. of pulley X rev. per min. X weight / 12

As an example, if the pulley of a band brake is 126.04 in. in diameter and makes 200 revolutions per minute, while a weight of 5 lbs. hung at end of the band just affects the speed, what is the H. P.?

3.1416 X 126.04 X 200 X 5 / 12 = 32,997.272 or approximately 33,000 ft.-lbs. per minute = 1 H. P.